The present invention relates to the general field of holographic storage systems and methods. More specifically the invention relates to a system and method for holographic storage with optical folding.
General holographic storage systems are discussed in xe2x80x9cHolographic Memoriesxe2x80x9d, by Demetri Psaltis et. al., Scientific American, November 1995, which is hereby incorporated by reference. Holography is also discussed in the text Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag which is hereby incorporated by reference. The basic principles of holography involve the recording of an interference pattern formed between two beams of light, referred to as an object beam and a reference beam. The object beam is encoded with data in a two dimensional pattern. The reference beam is used to form the interference pattern with the encoded object beam and is subsequently used to reconstruct the data by illuminating the recorded pattern.
There are several typical holographic storage geometries and imaging systems. A Fourier-plane, or 4-F, geometry is one such imaging system, where the spatial light modulator is Fourier-transformed-onto the holographic material and the reconstruction in turn is Fourier-transformed onto the detector array. In this architecture, a first fourier transform lens of focal length f1 is inserted between the spatial light modulator and the holographic storage media, and a second similar fourier transform lens of focal length f2 is inserted between the holographic storage media and the detector array. The spatial light modulator and the first principal plane of the first lens are separated by a distance f1, and the second principal plane of the first lens and holographic storage media are separated by a distance f1. On the detector side of the storage media, the holographic storage media and the second principal plane of the second lens are separated by a distance f2, and the first principal plane of the second lens and the detector array are separated by a distance f2. Thus, the principal planes of the two lenses are separated by the sum of their focal lengths, with a 2-D input and output plane located one focal length in front of and behind the lens pair. The magnification of the 4-F system is given by f2/f1. 4-F systems are described in Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., pages 28-30 and 429-431, copyright 2000, Springer-Verlag. The distance between the spatial light modulator and the first fourier transform lens, and the distance between the second fourier transform lens and the detector array is commonly referred to as the back focal length (or BFL). In certain designs, high performance fourier transform lens are utilized which operate with a high modulation transfer function. Such lenses provide a high signal-to-noise ratio at the storage medium and the detector array, resulting in a lower bit error rate at readout. However, the back focal length of such lenses is large, requiring a larger physical space in prior art 4-F systems.
FIG. 10 illustrates the basic setup of a typical prior art 4-F holographic system. The holographic storage system 700 includes a laser light source 710. The coherent light from the laser light source 710 is split into a reference beam and an object beam. The reference beam and object beam are directed to a holographic storage medium to record and recall holographic information. Light generated by laser light source 710 is directed to a beam splitter 715, such as a beam splitter cube, which splits the light from laser light source 710 into a reference beam 720 and an object beam 725. Reference beam 720 is reflected by a turning mirror 730 to a lens 735.
Object beam 725 is directed to a turning mirror 745 which directs the object beam to a pattern encoder 755, which encodes the object beam with data. The object beam is then directed to a holographic storage media 750 with a lens 780 of focal length f1. Pattern encoder 755 may be a spatial light modulator (xe2x80x9cSLMxe2x80x9d), or any device capable of encoding the object beam, such as a fixed mask, or other page composer. The pattern encoder 755 receives digitized data and imposes that pattern onto the object beam 725, such that the object beam 725 comprises an array of dark and light spots. The encoded object beam 725 is then directed to lens 780 that focuses the encoded object beam 725 to a particular site on the holographic storage media 750. Pattern encoder 755 is located a distance BFL1, (back focal length) from lens 780, and holographic storage media 750 is located a distance FFL1, (front focal length) from lens 780.
During readout of holograms previously stored in the holographic storage media 750, object beam 725 is blocked from transmission and a reference beam is projected at the same angle to the same spot on the holographic storage medium on which the desired information was previously stored. Diffraction of the reference beam with the previously stored hologram generates a reconstruction beam 782 that reconstructs the previously stored hologram. The reconstructed beam is transmitted towards imaging lens 784 directs and images the reconstruction beam onto the plane of the optical detector 786. Imaging lens 784 has a focal length f2. Imaging lens 784 is located a distance FFL2 from holographic storage media 750, and optical detector 786 is located a distance BFL2 from imaging lens 784. Optical detector 786 may be a conventional photodiode array, charge coupled device or other suitable detector array that transforms the encoded page into digitized data.
Although the prior art 4-F holographic systems offer the ability to accurately store holograms within a holographic storage media, there are disadvantages to existing systems. Existing systems require that the pattern encoder, first fourier transform lens, holographic storage media, second fourier transform lens, and optical detector be separated by a distance approximately f, requiring a certain physical space to house the optical components. Such systems often do not fit in standard drive envelopes.
Thus, there has been a need for improvements in the design of holographic storage systems. More specifically, there has been a need for more compact holographic storage systems.
The present invention provides a solution to the needs described above through a system and method/for holographic storage with optical folding.
In a first embodiment of the invention, the invention presents a system for recording and reading out holograms in a storage medium. The system comprises a pattern encoder, a first fourier transform lens with a focal length f1, a second fourier transform lens with a focal length f2, a detector array, a first prism located between the pattern encoder and the first fourier transform lens, and a second prism located between the second fourier transform lens and the detector array. The optical length between the pattern encoder and the first fourier transform lens through the first prism is equal to the back focal length BFL1, and the optical path length between the second fourier transform lens and the detector array through the second prism is equal to the back focal length BFL2.
A further embodiment of the invention presents a method for directing an object beam in a holographic system to a storage medium. The method comprises encoding an object beam with data utilizing a pattern encoder and directing the encoded object beam to a fourier transform lens with focal length f1 from the pattern encoder through a prism, where the optical path length between the spatial light modulator and the fourier transform lens through the prism is equal to back focal length BFL1. The encoded object beam is then fourier transformed to a holographic storage medium located a distance equal to front focal length FFL1 from the fourier transform lens.
An embodiment of the invention presents a further method for directing a reconstruction beam in a holographic system to a detector array. The method comprises directing a reconstruction beam from a holographic storage medium to a fourier transform lens with focal length f2 and fourier transforming the light beam to a detector array located a distance equal to back focal length BFL2 from the fourier transform lens. The light beam is directed from the fourier transform lens to the detector array through a prism, where the optical path length between the fourier transform lens and the detector array though the prism is equal to back focal length BFL2.